The invention relates to a belt retractor comprising a frame, a belt reel rotatably mounted about an axis in the frame, a control disc coaxial with the belt reel and a reduction gear unit which translates the angle of rotation of the belt reel into a reduced angle of rotation of the control disc. The invention relates furthermore to a method of controlling operation of an occupant restraint system.
In a belt retractor the total range of angular rotation of the belt reel is determined by the belt webbing in its fully coiled and fully uncoiled conditions. Within this range of angular rotation various relationships exist between the absolute angle of rotation of the belt reel and characteristic conditions of the belt system. Thus, zero angle of rotation (belt webbing fully coiled) is an indication that the belt is not in use. At maximum angle of rotation (belt webbing fully uncoiled) a switching action may be introduced to translate the belt retractor into an automatically blocked condition which is suitable for example for restraining loads or child seats. Between these extreme positions various characteristic conditions may be defined which, where necessary, in conjunction with other parameters, more particularly seating posture, seat occupancy and the like, may be assigned to particular switching conditions where activation of various security functions is expedient, it being especially desirable for the system to xe2x80x9cseexe2x80x9d the conditions in which activation of an air bag is to be inhibited, examples of which are the use of child seats, incorrect seated posture (e.g. leaning forward) and belt buckled with seat empty. Recognizing such conditions is only possible with adequate reliability when the angle of rotation of the belt reel can be sensed with high accuracy. It is known (e.g. DE 298 20 086.4) to translate the total range of angular rotation of the belt reel by means of a reduction gear unit to a single revolution of an indexing disc. Since, however, the range of rotation of the belt comprises a number of revolutions, each angle on the indexing disc corresponds to an accordingly greater angular range of the belt reel rotation so that the indexing positions cannot be sensed with the desired accuracy.
The invention provides a belt retractor of the aforementioned kind in which the absolute angle of rotation of the belt reel may be determined with high accuracy by uncomplicated technical means.
In accordance with the invention, the retractor comprises an incremental counter which is coupled to the belt reel. The incremental counter, on rotation of the belt reel, is incremented or decremented depending on the sense of rotation of the belt reel. The counter is calibrated by a sensor associated to the control disc so that the count of the incremental counter is representative of the absolute angle of rotation of the belt reel. By the count of the incremental counter the absolute angle of rotation of the belt reel is always represented with high accuracy. The accuracy of this representation depends solely on the resolution of the incremental counter. The incremental counter is formed preferably by an incremental scale non-rotatably coupled to the belt reel and a sensor fixed to the frame opposite the incremental scale. The pitch of the incremental scale then determines the resolution of the incremental counter. A suitable incremental detector may be achieved with optical or magnetic elements known in the art.
The control disc with the associated sensor has substantially the task of calibrating the incremental counter. In the simplest case the incremental counter is reset to zero when the belt webbing is totally coiled. By providing arcuate recesses on the periphery of the control disc, particular ranges of rotation of the belt reel may be detected which correspond to specific conditions of the belt retractor, for example a range of rotation of the belt reel that corresponds to the belt being buckled up.
The invention also provides a method of controlling operation of an occupant restraint system comprising a belt system including a belt retractor in accordance with the invention and at least one of the following components:
an inflatable protective cushion
a belt tensioner
a belt force limiter.
In the method in accordance with the invention, operation of these components of the restraint system is controlled as a function of the count of the incremental counter. The restraint function may be optimized by this method particularly effectively when the individual components are designed for operation in a plurality of stages. When, for instance, a seating posture is sensed which is out of position (OOP), then the air bag should only be inflated to a restricted degree or, indeed, not at all. Activating the air bag is to be totally inhibited when the count of the incremental counter indicates, where necessary in conjunction with other parameters, that the belt is being used to safeguard a child seat.
In one particularly advantageous aspect of the method, the second derivative of the count with respect to time furnishes an indication as to the rotary acceleration of the belt reel in the withdrawal direction. When a particularly high rotary acceleration is sensed, this is taken as an indication of a severe impact, i.e. the components of the restraint system then being activated to the highest stage.